do not discard const: keep signature

Here, while we do not mutate the struct itself, many other signatures
expect a non-const argument - possibly unnecessarily - so we opt to keep
the original signature by casting to non-const.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: mjg

pseudo-merge.c

@@ -644,7 +644,7 @@ static struct pseudo_merge_commit *find_pseudo_merge(const struct pseudo_merge_m
 	if (!pm->commits_nr)
 		return NULL;
 
-	return bsearch(&pos, pm->commits, pm->commits_nr,
+	return (struct pseudo_merge_commit *) bsearch(&pos, pm->commits, pm->commits_nr,
 		       PSEUDO_MERGE_COMMIT_RAWSZ, pseudo_merge_commit_cmp);
 }